1. Field of the Invention
This invention generally relates to a system for enabling an aircraft ground proximity warning system when an aircraft is on a final approach and, more particularly, to a system for providing an enabling envelope when an aircraft is on a final approach to an airport without the use of a flap position input signal.
2. Description of the Prior Art
Various systems are known that provide warning or advisory indications in the event of hazardous flight conditions. Among such systems are systems generally known as ground proximity warning systems for aircraft that serve to monitor the flight conditions of an aircraft and provide a warning if flight conditions are such that an inadvertent contact with the ground is imminent. Among the flight conditions monitored by such systems are radio altitude and rate, barometric altitude and rate, airspeed and flap and gear positions. The aforementioned parameters are monitored and an advisory indication or warning is generated when the relationship between the aforementioned conditions or parameters are such that a ground impact is likely to occur. Typical examples of such systems are disclosed in U.S. Pat. Nos. 3,715,718; 3,936,796; 3,958,218; 3,944,968; 3,947,808; 3,947,810; 3,934,221; 3,958,219; 3,925,751; 3,934,222; 4,060,793; 4,030,065; 4,215,334; 4,319,218, all assigned to the same assignee as the assignee of the present invention.
The above-described systems provide an advisory warning in the event of a hazardous flight condition. However, in optimizing the design of the warning criteria, it is difficult to arrive at warning criteria suitable for all phases of flight. In particular, it is desirable to alter the warning criteria in certain situations, such as, during a final approach phase, to reduce nuisance warnings. Several prior art systems utilize signals representative of aircraft configuration to control the alteration of the warning criteria when the aircraft is on a final approach.
In such configuration responsive systems, signals, such as, flap and gear position, are used to represent that an aircraft is on a final approach to an airport. In such systems, a 40.degree. flap position was used to indicate that the aircraft was on a final approach to an airport. The 40.degree. flap setting was used because it was a unique flap setting that was only used when an aircraft was on a final approach. However, certain newer aircraft now use a 20.degree. flap setting during the final approach, a setting that may be used during various phases of flight and not just final approach. This precludes the use of a flap position signal as an indication that the aircraft is on a final approach, because there is no way of distinguishing a 20.degree. flap setting used on final approach from a 20.degree. flap setting used in a different phase of flight.